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Kieran Kelly wrote:
>
> No definitely not. After using a theodolite several times the frustrating
> thing is that every time you touch the instrument the bubbles seem to move.
> Also if there is an error in the bubble it is quite a complicated procedure
> to allow for. The great advantage of a theodolite is that it gives azimuth
> as well as altitude and it is free from the "wobbly hands" of an observer.

First off, I didn't see the post to which Kieran is replying, perhaps
because it was HTML. I have a filter which routes all HTML email to
trash.

Now about theodolites. Kieran is correct, it's difficult to keep the
level bubble dead center. (The plate level on a Wild T3 moves its
bubble 2 mm per 7 seconds of angle.) So theodolites that read their
circles through optical micrometers have "compensators" to eliminate
vertical angle error due to misleveling. They work by slightly
deflecting the microscope image of the circle graduations. Some use a
pendulous prism suspended by Invar ribbons. Others use the reflective
surface of a fluid (not mercury) in a chamber. Either way, the
vertical circle reading is automatically corrected, provided the
misleveling is within the compensator's range.

Instead of automatic compensators, vernier theodolites have an index
level, visible in this picture:

http://www.sli.unimelb.edu.au/collection/item_details_30.html

You level the vernier frame with a fine adjustment knob before
each reading.

If a compensator or index level is out of adjustment, there's a
systematic error in vertical angles. However, the sign of this error
reverses when you reverse the instrument. (E.g., observe with the
vertical circle on your right instead of left.) Since averaging the
"face left" and "face right" observations cancels index error, it
needn't be eliminated, just kept reasonably small. The manual for the
Wild T2 (1-second theodolite) says the face left and face right
altitudes should agree within 60 seconds. Adjustment merely requires
opening a cover and turning a screw.

Simple instruments have fixed verniers for the vertical circle. With
these, the whole instrument must be carefully leveled before measuring
a vertical angle. Any error due to misleveling is not reversed when
you change face. However, index error due to a misadjusted vernier
*is* reversed, so it's still beneficial to change face.

   

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